This invention relates to sensors, and, more particularly, to the processing of sensor signals to selectively suppress irregularities resulting from detector element nonuniformity while maintaining a proper representation of objects in the sensor field of view.
A "sensor" (as used herein) is a device having an array of detector elements and associated optics that produces an image signal of a scene viewed by the detector elements. There are usually a large number, such as a 256.times.256 array, of detector elements in the sensor, and the matrix of detector output signals becomes the output signal of the sensor. The detector elements may be made sensitive to various types of energy, and in a common situation the detector elements are sensitive to infrared radiation. The image signal produced by the sensor is typically analyzed for various types of information, such as, for example, the presence of a target object in the field of view of the sensor.
Ideally, each of the detector elements of the array would operate in an absolutely identical manner, so that each detector element would produce the same output signal in response to a particular input signal, over the entire range of the sensor sensitivity. Thus, for example, if the sensor were directed at a featureless scene, under the ideal situation the output of each detector element would be the same. If there are nonuniformities between the outputs of the various detector elements when viewing a featureless scene, the nonuniformities might erroneously be interpreted as features in the field of view of the sensor. This same conclusion is applicable when the scene is not featureless, and it is therefore possible to confuse detector nonuniformities with features.
The ideal case of uniformity in all of the detector element outputs cannot be realistically expected in large element arrays for a variety of reasons. Techniques have therefore been developed to suppress detector-based nonuniformities in the sensor output. For example, U.S. Pat. No. 4,975,864 discloses scene-based nonuniformity compensation that serves to reduce stationary, as contrasted to moving, nonuniformities in the sensor output signal. Stationary nonuniformities are those whose relative position does not change from image to image, and may be due to detector-element nonuniformities. Moving nonuniformities, on the other hand, are features present in the scene viewed by the sensor which exhibit their own motion or motion due to movement of the sensor.
Conventional nonuniformity compensation techniques, while serving a valuable function, may have significant limitations. One such limitation is the possible mistaking of a stationary feature in the viewed scene as detector-based nonuniformity, and the resulting elimination of that feature from the sensor output signal. That is, when a feature is absolutely stationary in the viewed scene, the nonuniformity compensation processor may conclude that the feature is an artifact of detector nonuniformity, and act to progressively reduce the intensity of the feature.
There is a need for a technique whereby stationary objects in the sensor field of view may be differentiated from detector-based nonuniformities. The true nonuniformities are then removed from the sensor output signal in the conventional manner, while the stationary features are permitted to remain in the sensor output signal for further analysis. This differentiation and processing technique must not interfere with the accurate positioning of the features, when the image is eventually analyzed in detail for the presence of targets. The present invention fulfills this need, and further provides related advantages.